Switching devices are commonly used in various applications, for example in power supply units, in oscillators, in inverters, in power amplifiers and similar. In a particular switching device, two electronic switches, typically two transistors, are connected in an half-bridge configuration. Each of the two transistors is alternatively switched-on and switched-off; when one transistor is on, the other transistor is off and vice versa.
A problem in half-bridge switching devices consists of a phenomenon known as cross-conduction. This is due to the fact that, in general, the turn-off time of the transistor which is on is higher than the turn-on time of the transistor which is off, so that at each switching of the two transistors there is an interval of time in which both transistors are on. This creates a short-circuit at the terminals of the half-bridge, which generates current peaks that can damage the transistors.
In order to prevent this problem, the transistors are typically controlled in a disoverlap manner, in particular, each transistor is switched on with a preset delay, called dead time, with respect to the instant in which the other transistor is switched off, so as to ensure that the two transistors are never both on at the same time.
A known solution for generating a signal with a duration equal to the dead time consists in employing a transient charging (or discharging) phenomenon in a circuit including a capacitor and a resistor, or an RC circuit, which time constant is proportional to the capacitance and the resistance of the capacitor and of the resistor, respectively. The RC circuit is typically integrated in a chip of semiconductor material wherein the switching device is also made; consequently, these parameters are difficult to control with accuracy and are subject to wide variations on the basis of temperature.
Consequently, the capacitor and the resistor need to be dimensioned so as to ensure that in any case the dead time is sufficiently long to prevent the two transistors from being on at the same time. This increases the duration of the interval of time in which the switching device is not active (with both transistors off) and does not allow the use of the device at high switching frequencies. Additional drawbacks of such structure consists in that the RC circuit is very cumbersome and occupies a considerable area on the semiconductor chip.
A different solution consists in using a digital circuit comprising a clock signal generator, made for example by means of a quartz oscillator, which is also used to switch the two transistors; this allows an extremely accurate dead time to be obtained. However, since the known clock signal generators operate with a frequency at most equal to several MHz and the duration of the on state and the off state of the transistors is generally higher by several orders of quantities than the duration of the dead time, such a solution can only be used at rather low switching frequencies, for example not higher than several tens of KHz.